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FAQ02316 of Safety Limit Switches FAQ

FAQ No. FAQ02316

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What is negative operation and positive operation?


(A) Negative operation(B) Positive operation(C) Combined operation
SafetyNever use negative-operation switches alone in safety applications.Positive-operation switches are recommended when used alone as the switches offer a higher level of safety than negative-operation switches.Switches in combined operation offer an even higher level of safety than positive-operation switches alone.
CategoryCategory B or 1
(using approved parts)
B, 1, 2, 3, 4B, 1, 2, 3, 4
Normal operationAbnormal operationNormal operationAbnormal operationNormal operation
Contacts closed
(door closed)
a) No reset due to contact welding
(door open)
Contacts closed
(door closed)
a) Contact not open due to cam abrasion
(door open)
Contacts closed (door closed)
Contacts open
(door open)
b) No reset due to spring damage
(door open)
Contacts open
(door open)
b) Contact not open due to improper cam position (door open)
Contacts open (door open)
Opened by built-in spring.Opened directly by externally operating unit like cam or dog.Opened by a negative- and positiveoperation combination.
NO contactsNC contacts
NO and NC contacts
ProsThe negative operation is a fail-safe operation that ensures safety in case of cam abrasion, improper cam positioning or unexpected cam removal.The actuator forcibly opens contacts if a contact welds or a spring is broken.A negative- and positive-operation combination eliminates the disadvantages of both modes.
ConsThe actuator may move accidentally with unexpected force and close the contacts. The result may be a relatively dangerous situation.There is a danger that contacts may close due to cam abrasion, improper cam positioning or unexpected cam removal.The safety switch circuit may work normally for a while if one of the switches fails to operate.

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